1. Field of the Invention
The present invention relates to a solid-state image sensing device and, more particularly, to a solid-state image sensing device including analog to digital converting units.
2. Description of the Related Art
A CMOS image sensor including an A/D converter (to be referred to as a “column ADC” hereinafter) for performing analog to digital (A/D) conversion for each column of unit pixels arranged in a matrix topology is used as a solid-state image sensing device. As an A/D conversion method, a plurality of comparators are used to respectively compare pixel signals with reference signals which have different rates of change with respect to time, thereby performing A/D conversion based on comparison times (see Japanese Patent Laid-Open No. 2007-281987 (literature 1)).
Furthermore, there is provided a method of selecting, among a plurality of reference signals each with a ramp voltage, a reference signal appropriate for a pixel signal based on a pixel signal level determination result, and comparing the selected reference signal and the pixel signal with each other, thereby performing A/D conversion based on a comparison time (see Japanese Patent Laid-Open No. 2006-352597 (literature 2)).
For the method using the plurality of comparators described in literature 1, however, the circuit scale is large. Furthermore, literature 2 does not describe a method of supplying a plurality of reference signals when comparing the pixel signal with the reference signal which has been selected from the plurality of reference signals each with a ramp voltage.